KIEV (Reuters) - Ukraine said on Tuesday a group of Russian servicemen had crossed into Ukrainian territory on a "special mission," contradicting Russian media which cited a defense ministry source in Moscow as saying they got there by accident.

"This wasn't a mistake, but a special mission they were carrying out," Ukrainian military spokesman Andriy Lysenko said in a televised briefing.

He also said separatists were attacking the southeastern border town of Novoazovsk "at this very minute" and Ukrainian forces had destroyed 12 armored infantry vehicles in the area.

Twelve Ukrainian service personnel had been killed in fighting in the past 24 hours, he said.